Summary

In unconventional reservoirs the occurrence of marls is a significant risk to production, and a major challenge is locating and quantifying them. Marl thickness can be below seismic resolution, so advanced characterization techniques are required. Stochastic inversion of seismic can be an efficient method to predict key rock properties, such as lithofacies variability. This approach is based on a comparison of real and synthetic seismic data. By creating a large set of 1D Earth models (pseudo-wells) derived from real wells, a 3D seismic inversion can predict marl distribution and percentage, thus improving reservoir delineation and enhancing well placement.

Introduction

The Otter Park Formation of the Horn River Basin, northeastern British Columbia, Canada, is an unconventional reservoir that holds a significant amount of self-sourced gas trapped in a tight shale lithofacies. The shale succession presents slow lateral variations in terms of thickness and rock properties (Gal and Jones, 2003) and some wells have encountered marly laminated mudstone deposits encased in the shales. These marl deposits constitute lean non-reservoir zones which reduce anticipated reserves and therefore need to be mapped. An additional complicating factor is that the individual thicknesses of the marl units are below seismic resolution, i.e no individual seismic event can be uniquely attributed to the presence of marl.

However, the elastic properties of the marls are sufficiently different from the encasing shales to cause measurable effects in the composite seismic reflection response. This is the key to their identification using the HitCube rock property prediction workflow. The HitCube is a geologically-driven stochastic inversion method that works well in areas with limited well control. The method takes into account non-unique solutions and outputs rock property probability volumes with associated uncertainties.